1. Field of the Invention
The present invention relates to an image forming apparatus that supplies a sheet on which an image is formed to a ring bookbinding apparatus. Particularly, the present invention relates to a ring bookbinding technique for binding a book by threading rings through punch holes of sheets.
2. Description of the Related Art
Conventionally, a bind processing apparatus that aligns sheets to make a sheet bundle, attaches a ring type binding member to punch holes of the sheets by pushers, and ejects a bound booklet to a stack tray is proposed (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2005-138549 (JP 2005-138549A)).
When a plurality of booklets are made by such a ring bookbinding, a buffer device, which is located at the upstream side of a bookbinding unit, temporarily stores a plurality of sheets in order not to supply sheets for a next sheet bundle while the bookbinding unit is binding a previous sheet bundle. Then, when the bookbinding unit finishes the bookbinding process, the sheets stored in the buffer device are supplied to the bookbinding unit in piles. This enables to execute the bookbinding process without decreasing productivity.
However, since a plurality of sheets are conveyed in piles when the sheets are stored in the buffer device according to the above mentioned method, the sheets may stick mutually when the sheets are made from sticky material (OHP sheets, coated sheets, etc.). The mutual stick of the sheets causes misalignment of the sheet bundle, which causes a fault in a subsequent bookbinding process. Therefore, a sheet made from such material is prohibited to be buffered.
In particular, when an OHP sheet or a coated sheet is used for the first page (for example, a cover or a back cover) of a sheet bundle to be bound, sheets for a next sheet bundle cannot be printed and stored into the buffer device while binding a previous sheet bundle. Therefore, the conveyance interval of sheets must be extended, which decreases productivity.